Effects of 3,4-diaminopyridine on myoplasmic calcium and phosphoinositide hydrolysis in frog sartorius muscle fibers

Using Ca²⁺-selective microelectrodes, the effects of 3,4-diaminopyridine (DAP) on myoplasmic calcium ([Ca²⁺](i)) were studied in frog sartorius muscle fibers. DAP induced an increase of [Ca²⁺](i) in a concentration-dependent manner. DAP 1 mmol · L^-1 increased the [Ca²⁺](i) from control 166 ± 41 nmol · L^-1 to 416 ± 69 nmol · L^-1 (n = 10). In the absence of external calcium, DAP still enhanced [Ca²⁺](i). [Ca²⁺](i) of the fibers bathed in Ca²⁺-free Ringer's solution containing DAP 1 mmol · L^-1 was 152 ± 43 nmol · L^-1, which was significantly higher than 77 ± 35 nmol · L^-1 of [Ca²⁺](i) of the fibers in Ca²⁺-free Ringer's solution. In addition, DAP promoted the hydrolysis of phosphoinositides, and DAP induced hydrolysis was more in the presence of external calcium. It is suggested that, through enhancing the hydrolysis of phosphoinositides, DAP released Ca²⁺ from intracellular Ca²⁺ store in frog sartorius muscle fibers.